1. Field of the Invention
The present invention relates to a method for removing sulfur dioxide from an exhaust gas and more particularly to a method for removing sulfur dioxide from an exhaust gas in the form of calcium sulfate (gypsum).
2. Description of Prior Art
There have been proposed various methods of removing sulfur dioxide from an exhaust gas, such as an engine exhaust gas, in the form of calcium sulfate. In a typical wet-type process, the exhaust gas is contacted with an alkali sulfite-containing aqueous solution, whereby sulfur dioxide contained in the exhaust gas is absorbed in the aqueous solution to react with the alkali sulfite, producing alkali bisulfite. Slaked lime (calcium hydroxide) or limestone (calcium carbonate) is added to the aqueous alkali bisulfite solution to form calcium sulfite which is then oxidized into calcium sulfate for removal, as shown by reaction formulae (1) through (4) below, where sodium sulfite is the alkali sulfite by way of example. EQU Na.sub.2 SO.sub.3 + SO.sub.2 + H.sub.2 O .fwdarw. 2NaHSO.sub.3 ( 1) EQU 2naHSO.sub.3 + CaCO.sub.3 .fwdarw. CaSO.sub.3 + Na.sub.2 SO.sub.3 + CO.sub.2 + H.sub.2 O (2)
alternatively, EQU 2NaHSO.sub.3 + Ca(OH).sub.2 .fwdarw. CaSO.sub.3 + Na.sub.2 SO.sub.3 + 2H.sub.2 O (3) EQU caSO.sub.3 + 1/2 O.sub.2 .fwdarw. CaSO.sub.4 ( 4)
In an wet process which employs the alkali sulfite-containing aqueous solution to remove sulfur dioxide from the exhaust gas by reaction with alkali sulfite to form bisulfite, part of the alkali sulfite inevitably reacts with oxygen and other gases contained in the exhaust gas and is oxidized into alkali sulfate resulting in a buildup of alkali sulfate in the alkali bisulfite solution. When such a buildup of alkali sulfate takes place to a significant degree, the quantity of alkali sulfite in the alkali sulfite-containing aqueous solution, available for reaction with sulfur dioxide decreases thus reducing the rate of sulfur dioxide absorption, and efficiency. The alkali sulfate, which accumulates as a by-product, is only slightly reactive with slaked lime or limestome which is added to the alkali bisulfite-containing aqueous solution for the purpose of forming calcium sulfite. The alkali sulfate thus, must be removed in advance from the alkali bisulfite solution by a suitable method. To facilitate removal of the alkali sulfate, for example, sulfuric acid and calcium sulfite are introduced into the alkali bisulfite-containing solution from an external source to convert the alkali sulfate into calcium sulfate, as shown by reaction formulae (5) and (6) below, where sodium sulfate is used as the alkali sulfate. EQU H.sub.2 SO.sub.4 + CaSO.sub.3 + H.sub.2 O .fwdarw. CaSO.sub.4 + SO.sub.2 + 2H.sub.2 O (5) EQU so.sub.2 + na.sub.2 SO.sub.4 + CaSO.sub.3 + 3H.sub.2 O .fwdarw. CaSO.sub.4 + 2NaHSO.sub.3 + 2H.sub.2 O (6)
the introduction of sulfuric acid and calcium sulfite from an external source to the alkali bisulfite-containing aqueous solution adds an undesirable complication to the process.
The present inventors have disclosed in their copending application, U.S. Ser. No. 440,381, filed Feb. 7, 1974 a method for easy removal of sulfur dioxide from exhaust gas which overcomes the aforementioned shortcomings of the prior-art methods and does not reduce in the sulfur dioxide removal efficiency. The method involves contacting a sulfur dioxide-containing exhaust gas with an aqueous solution containing at least one organic acid salt, expressed by the formula RCOOM, wherein R represents, methyl, ethyl or propyl and M represents an alkali metal or NH.sub.4. The salt reacts with the sulfur dioxide contained in the gas to form a sulfite. In accordance with the method of U.S. Ser. No. 440,381, the sulfur dioxide-containing exhaust gas is brought into contact with an aqueous solution containing an alkali salt of the acid, and the sulfur dioxide is removed from the exhaust gas by dissolution in the aqueous solution in the form of alkali sulfite. After oxidizing the alkali sulfite to alkali sulfate, a calcium compound such as slaked lime or limestone is brought into contact with the aqueous solution to convert the alkali sulfate into calcium sulfate, which is separated by filtration from the solution in a subsequent recovery step. The remaining liquor or filtrate containing the organic acid salt is recycled for use as the aqueous scrubbing solution. The present inventors have discovered that a small quantity of calcium sulfate remains in the filtrate reacts with alkali sulfite which is produced by contact of the exhaust gas with the filtrate, forming calcium sulfite which is less soluble than calcium sulfate and tends to gradually deposit on the gas-absorber and associated apparatus as scale. The chemical reaction which results in the formation of calcium sulfite is shown by reaction formula (7) below, where sodium sulfite is the alkali sulfite for purposes of illustration. EQU CaSO.sub.4 + Na.sub.2 SO.sub.3 .fwdarw. CaSO.sub.3 + Na.sub.2 SO.sub.4 ( 7)
the formation of scale on the reactor walls will increase the pressure drop in the absorption tower and may result in blockage of auxiliary piping which can cause various trouble during operation, maintenance and management of the plant. There exist various chemical and mechanical methods for the removal of such scale including dissolution or washing by or with chemicals and continuous or periodical mechanical removal by scraping or water under pressure. In the chemical method, it is necessary to either suspend operation of the reactor, at least during the removal, or to use a reserve reaction tower to allow continuous operation, resulting in high operational costs or investment. With mechanical removal, it is difficult to remove the scale to a satisfactory degree especially with a reactor of a large size.
As will be understood from the foregoing description, there exists a need for an efficient method of removing sulfur dioxide from the exhaust gas in the form of calcium sulfate which does not result in formation of scale within the reactor.